4-Nitroestrone 3-methyl ether been shown in this laboratory to be a potent growth inhibitor of the MXT mouse mammary tumor and the DMBA-induced rat mammary tumor in intact and ovariectomized animals. It is the purpose of this proposal to obtain information which may aid in the determination of the mechanism by which this non-toxic compound brings about regression in hormone dependent mammary tumors. Since 4-nitroestrone 3-methyl ether was initially designed as a specific inhibitor of estrogen sulfotransferase, experiments are planned which will indicate whether the blockage of this mammary tumor enzyme is related to tumor regression, whether the receptor mechanism is involved, or whether tumor regression is the result of periferal influences of this agent. Furthermore since this polar A-ring substituted estrogen exhibits only a modest capacity to enter target tissue, new less polar derivatives will be synthesized which are more amenable to tissue uptake, are capable of inhibiting the sulfotransferase, but are not able to be dealkylated in vivo to form metabolites which interact with receptors. 4-Nitroestrone 3-methyl ether and other A-ring substituted estrogens [in general the 2 and 4-substituted estrogens, their 3-O-alkyl ethers or 3-O-(Alpha, Beta, Gamma-hydroxyalkyl) ethers and 2 or 4-substituted estra-1,3,5(10)-triene-17-one compounds] will be tested in various experimental tumor systems (e.g. DMBA-and NMU-induced breast cancer in rats, the MXT hormone dependent ductal carcinoma, the 16/c and 13/c hormone independent mouse mammary tumors, the MX-1 human tumor and MCF-7 human breast cancer cells in athymic mice).